Optimized Setup and Protocol for Magnetic Domain Imaging with In Situ Hysteresis Measurement

The overall goal of this experiment is to demonstrate a novel dynamic magnetic domain imaging system with In Situ BH measurements, and how to use it to link the magnetic domain wall movement with the BH curves. This method can help answer key questions in the relationship between microstructure and magnetic properties in paramagnetic materials such as ferritic steels. The main advantage of this technique is that it enables inter QBH measurements without interrupting the dynamic domain imaging.

The first task is to prepare the rig used for imaging. This is an example of a rig that is ready for use in an experiment. The metallographic samples are visible in this side view.

Also visible are the excitation coils. More details can be seen in this schematic. The sample consists of two parts, A and B.Part A is partially encased in a polished surface for domain imaging.

Part B is wound by a pick-up coil. The rig consists of three parts. The front plate holds part A.The sample holder holds part B.A back plate holds a hall sensor near the pick-up coil.

Here are the disassembled elements of the rig before the samples are in place. For an experiment, begin by preparing the samples. Machine the two U-shaped parts, A and B from the steel of interest.

Note that the two are slightly different, with part A having chamfering on a wider bar. Focus on part A and produce a transparent mount using hot compression mounting. The final thickness of the mount should be five to 10 millimeters greater than the height of the sample.

Next, work with the mounted sample at a grinding machine. Orient the sample so it's open side faces 320 grit silicon carbide paper. Proceed with the grinding process.

Stop when the legs of the sample are revealed on the surface. Re-orient the sample to grind the opposite side with the flat part of the U.Start grinding again and check frequently. Stop when the rectangular surface of the sample is revealed.

Use calibers to measure the length of the revealed sample. It should initially be about 23 millimeters, reflecting the chamfer. Continue grinding and measuring the revealed portion of the sample.

Stop as soon as the length is measured to be 25 millimeters, the same as part B.Polish the sample before moving on to etch it. Starting with polished sample, use a cotton swab dipped in two percent nital and etch for one to five seconds until the surface turns matte. When done, rinse the sample with water and blow it dry.

Take the sample to an optical microscope to check that the microstructure is clearly visible. Then polish the sample with a one micrometer diamond polishing agent to remove the etched surface. Repeat the etch, inspect, polish sequence four to six times.

This is the final result after polishing the surface in a luminous suspension for two minutes. Here are parts A and B after they have been prepared for the experiment. Part B has a 50 turn flux density measurement coil on it's longest side.

With the components ready, construct the domain imaging rig. Place the front plate on a flat surface. Place the mounted sample, part A over the hole in the plate and fit it inside.

Apply hot melt from a glue gun around the circumference of the mounted sample to hold it in place. Next, put part A aside to focus on part B.Get the sample holder and part B.Insert part B through the excitation coils into the bottom of the holder. It should protrude about one millimeter from the top.

Now get the back plate, which has a hall sensor on the side that faces the sample. Align the hall sensor with the sample in the holder. Then loosely tighten the nuts to hold the two together.

Retrieve the front plate with part A.The front plate now has to be connected to the rest of the rig. To help with assembly, connect the excitation coils to a current source and apply current. Align the open end of part A with the open end of part B visually and with the feedback of the electromagnet.

Bolt the top plate to the sample holder and tighten the bottom nuts to complete the assembly. To perform dynamic imaging, have ready a microscope with an attached high-speed video camera. Turn attention to preparing the sample.

For use with the microscope, affix the sample rig to a glass slide with modeling clay and level it. Draw a single drop of the ferro fluid with a pipette and apply it on the sample surface. Next get a clean glass microscope slide and place it on the sample.

Slowly pull the glass slide off the sample surface to leave a thin, uniform, semi-transparent layer. Place the sample rig on the microscope stage. Next make the necessary connections for the In Situ domain imaging system.

Referring to this schematic, the main components are the camera, a custom BH analyzer, a data acquisition breakout box, and a computer. Connect the sensor excitation coils to the power output of the BH analyzer. Connect the hall sensor to the H input channel of the BH analyzer and the B sensor coils to the B input.

The H and B outputs of the BH analyzer connect to the analog input channels of the data acquisition box. Connect the sync in and trigger of the camera to the sinc out and trigger of the data acquisition box, respectively. The computer connects to the camera, data acquisition box, and the BH analyzer for control and data storage.

Within the BH analyzer software, set the necessary test parameters. In the data acquisition software, set the data sync parameters for the experiment. Use the BH analyzer to apply a one hertz excitation sinusoidal current to measure the major loop.

Check that the measured BH loop displayed is roughly as expected in terms of the coercive field, remnants, saturation, and other values. This check can indicate if there is a problem with the coupling between parts A and B.If the loop is as expected, trigger the camera to record and monitor the BH loop. This is an example of domain processes recorded using the domain imaging system over three cycles of a BH loop.

Each cycle represents one second. The recording reveals domain rotation and 180 degree domain walls interacting with domain wall pinning features. The sample is laboratory steel with extra low carbon and copper sulfide precipitants.

This is an In Situ measured BH loop. The numbers indicate the high speed camera frame associated with that point in the cycle. Starting with the first frame, observe the 180 degree domain walls in the region labeled A.The magnetic field points right with an uncertainty of plus or minus 10 degrees.

Proceeding up the BH curve, by frame 50 the domain walls are 90 degree. Continuing along the curve, the 90 degree domain walls revert to 180 degree domain walls between frames 225 and 250. Following the procedure, further microstructure characterization can be performed to link the memory movements to specific microstructure features such as the grain boundaries of precipitates or the domain's reaction to the crystalized graphic orientation of the grains.

After it's development, this technique paved the way for the researchers in the field of magnetic non-destructive testing, and the magnetic materials to expose fundamental links between the memory movement, microstructure, and magnetic properties. After watching this video you should have a good understanding of how to obtain optimal automated patterns in structural steels by a better technique, and how to realize In Situ BH measurement with dynamic domain imaging.